Niobate-based octahedral molecular sieves

Abstract

Niobate-based octahedral molecular sieves having significant activity for multivalent cations and a method for synthesizing such sieves are disclosed. The sieves have a net negatively charged octahedral framework, comprising niobium, oxygen, and octahedrally coordinated lower valence transition metals. The framework can be charge balanced by the occluded alkali cation from the synthesis method. The alkali cation can be exchanged for other contaminant metal ions. The ion-exchanged niobate-based octahedral molecular sieve can be backexchanged in acidic solutions to yield a solution concentrated in the contaminant metal. Alternatively, the ion-exchanged niobate-based octahedral molecular sieve can be thermally converted to a durable perovskite phase waste form.

1. A method for removing a contaminant metal ion from a contaminated solution, comprising the steps of:
 adding a niobate-based octahedral molecular sieve, having a general chemical formula Na 2 Nb 2−x O 6−x (OH) x .H 2 O wherein x=0.04–0.4 and M=Ti, Zr to the contaminated solution containing the contaminant metal ion; 
 exchanging the alkali cation Na for the contaminant metal ion to form a contaminant metal ion-exchanged niobate-based octahedral molecular sieve and a contaminant-depleted solution; and 
 separating the contaminant metal ion-exchanged niobate-based octahedral molecular sieve from the contaminant-depleted solution. 
 
     
     
       2. The method of  claim 1 , wherein the contaminant metal ion is selected from the group consisting of La 3+ , Sr 2+ , Pb 2+ , Ba 2+ , Zn 2+ , Cd 2+ , Co 2+ , Cr 3+ , Ni 2+ , Ca 2+ , Mg 2+ , Cs + , K + , and Li + . 
     
     
       3. The method of  claim 1 , further comprising:
 backexchanging the contaminant metal ion-exchanged niobate-based octahedral molecular sieve in an acidic solution to yield a concentrated solution of the contaminant metal ion. 
 
     
     
       4. The method of  claim 1 , further comprising:
 heating the contaminant metal ion-exchanged niobate-based octahedral molecular sieve to form a contaminant metal-loaded perovskite. 
 
     
     
       5. The method of  claim 4 , wherein the heating step comprises heating the contaminant metal ion-exchanged niobate-based octahedral molecular sieve to greater than about 550° C.